A hydrogen-generation assembly is an assembly that includes a fuel processing system that is adapted to convert one or more feedstocks into a product stream containing hydrogen gas as a majority component. The produced hydrogen gas may be used in a variety of applications. One such application is energy production, such as in electrochemical fuel cells. An electrochemical fuel cell is a device that converts a fuel and an oxidant to electricity, a reaction product, and heat. For example, fuel cells may convert hydrogen and oxygen into water and electricity. In such fuel cells, the hydrogen is the fuel, the oxygen is the oxidant, and the water is the reaction product. Fuel cells typically require high purity hydrogen gas to prevent the fuel cells from being damaged during use. The product stream from the fuel processing system of a hydrogen-generation assembly may contain impurities, illustrative examples of which include one or more of carbon monoxide, carbon dioxide, methane, unreacted feedstock, and water. Therefore, there is a need in many conventional fuel cell systems to include suitable structure for removing impurities from the impure hydrogen stream produced in the fuel processing system and/or from the hydrogen-containing fuel stream for a fuel cell stack.
A pressure swing adsorption (PSA) process is an example of a mechanism that may be used to remove impurities from an impure hydrogen gas stream by selective adsorption of one or more of the impurities present in the impure hydrogen stream. The adsorbed impurities can be subsequently desorbed and removed from the PSA assembly. PSA is a pressure-driven separation process that typically utilizes a plurality of adsorbent beds. The beds are cycled through a series of steps, such as pressurization, separation (adsorption), depressurization (desorption), and purge steps to selectively remove impurities from the hydrogen gas and then desorb the impurities.
Many hydrogen-generation assemblies include a heating assembly that combusts at least one fuel stream with air to produce a heated exhaust stream for heating at least a portion of the hydrogen-generation assembly. The fuel streams may come from a variety of sources, including the PSA assembly. However, PSA assemblies are operated in PSA cycles that may result in the production of exhaust, or byproduct, streams having varying and intermittent flows and/or varying fuel values, or only intermittently discharge byproduct streams. When used as a fuel stream for a heating assembly, this intermittency, variation, and/or interruption in flow rate and/or fuel value may produce inconsistent, often unpredictable, results in the heating assembly, such as during periods of no fuel, periods of insufficient fuel, periods in which the fuel streams have variable fuel values, etc. As a result, it may be difficult for the heating assembly to maintain a selected component, or region, of the hydrogen-generation assembly at a desired temperature or within a desired, or selected, temperature range. Similarly, at times, the PSA assembly may not be producing sufficient, or any, exhaust stream to maintain a flame or other ignition source of a heating assembly in operation.